Author: Marie Hartley, Staff Writer, 2009.
Human toxocariasis is caused by infestation of the larvae of two widespread roundworms, Toxocara canis and Toxocara cati. Dog and cat intestines are the reservoir for T. canis and T. cati respectively. T. canis is more commonly responsible for human disease. Human toxocariasis is widespread and is usually symptom-free. Almost 14% of the United States population is estimated to be infected, with even higher rates in rural areas and tropical countries.
Most eggs enter the environment via the faeces of puppies and lactating dogs. Humans usually become infected by ingesting eggs from the environment. The disease commonly affects young children with a history of eating soil, exposure to dogs, and those living in poor socioeconomic conditions.
The ingested toxocara eggs develop into larvae, which penetrate the gastrointestinal mucosa. The larvae are then carried in the bloodstream to the liver, lungs, and other organs, where they can remain for long periods. A specific type of white blood cells, called eosinophils, accompany the larvae and infiltrate the organs and tissues in response to the infection. Raised levels of eosinophils can usually be detected in the blood.
Mild infections with toxocaral larvae often produce no symptoms. When symptoms are present, a variety of clinical syndromes have been described.
|Clinical syndrome||Clinical features|
|Visceral larva migrans (VLM)||The liver and lungs are most commonly involved producing abdominal pain, decreased appetite, enlarged liver, coughing, wheezing, restlessness, fever, and asthma. Skin disease occurs in around one in four cases and includes rashes, urticaria, and hypodermic nodules (small lumps below the epidermis).|
|Ocular larva migrans (OLM)||Migration of T. canis larvae to the eye causes loss of vision.|
|Common toxocariasis||Usually affects adult patients living in rural areas of the Midi-Pyrénées region of France. Presents with chronic lack of energy and weakness associated with digestive disorders. Blood tests show raised levels of eosinophils and total IgE. Skin manifestations such as itch, eczema/dermatitis, and urticaria are sometimes present.|
|Covert toxocariasis||Non-specific symptoms such as cough, abdominal pain, headache, sleep disorder, vomiting, enlarged liver, and occasionally a rash. Blood tests show raised eosinophils in around half of all cases.|
VLM and OLM are usually more severe, whereas common and covert toxocariasis are less severe and are spontaneously cured in most cases.
Skin manifestations are relatively common in patients with toxocariasis, particularly in VLM and common toxocariasis. Occasionally skin manifestations may be the sole feature of the disease. Given the high prevalence of toxocariasis worldwide, the diagnosis should be considered in any case of undiagnosed suspicious skin lesions that have been recurring for several months or years, such as:
Although the definitive diagnosis is based on detection of Toxocara larvae in tissue samples, obtaining biopsy material containing larvae can be difficult and is usually not necessary. The diagnosis is usually based on clinical features and results of blood tests:
The ELISA test can be modified to confirm OLM using fluid samples from inside the eye.
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